I know your fish are small just now but they need lots of room for them to grow healthily.
goldfish need a big tank when small no less than 20 gallons, yes thats big but goldies are huge waste producers and need lots of free space to swim. fancy goldfish can get away with 10 gallons as they dont swim as much.
I understand that not everyone can rush out and get a big tank but anything smaller than a 20 gallon for these two wouldnt last very long before needing to be upgraded fast as you have already found out.
filtration is very important for goldie health as well. You will need a filter that can turn over the water 10 times per hour.
dechlorinated water is also vital as well as test kits for ammonia, nitrite, nitrate and ph.
as you can see its not a cheap hobby if done correctly.
in the meantime you will need to do 100% water changes each day to keep the tank ammonia free. the filter you get must do 10x filtration. hope some of that helps

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